The present invention relates to third generation cellular systems having downlink congestion control.
The architecture of third generation cellular systems based on Universal Mobile Telecommunications System (UMTS) standards includes radio network subsystems (RNSs) which comprise radio network controllers (RNCs) and “Node-Bs.” The Node-Bs are also referred to as base stations (BSs). The UMTS standards utilize a layer model developed by the International Organization for Standization (ISO). The layer model is the Open Systems International (OSI) seven (7) layer model which provides a common basis for the development of standards for interconnecting systems.
The specific architecture of the layers may vary according to the systems being interconnected. For example, there are three layers in the interface (Uu interface) of a radio network subsystem (RNS) and a user equipment (UE). The three layers perform different functions, but collectively control data transmitted from a RNS to a UE (i.e. the control plane). The first layer, layer 1, is a physical layer which codes, interleaves and multiplexes or otherwise transmits data across physical channels for delivery from a BS to a UE. Layer 1 processing may be performed by the BS. The second layer, layer 2, is a data link layer which ensures that data is properly transmitted by the physical layer. The third layer, layer 3, is a network layer which establishes and terminates connections between upper layers of the RNS and UE.
In order to serve the network-to-user (downlink) connections, a RNC within the RNS performs layer 2 and 3 processing on the incoming data from the core network, and directs blocks of data called “transport blocks” toward the BS serving the user that is to receive the information. The BS performs physical layer processing of the transport blocks (i.e. coding, interleaving, multiplexing) followed by modulation and transmission of the signals.
The RNC which is directly connected to the core network and which performs layer 2 and 3 processing for a given user is said to be the serving RNC (SRNC) of this user. This RNC may not be the same as the RNC that is connected to the BS serving the user, which is said to be the controlling RNC (CRNC) of the BS. When the SRNC and the CRNC are not the same for a given user, the transport blocks of this user are routed transparently by the CRNC to the BS serving the user.
Normally, the BS should process all transport blocks it has received from its CRNC. However, its resources in terms of processing power and transmission power are limited and it may occasionally happen that the BS is unable to process all the transport blocks received from the CRNC and/or transmit the modulated signals with a power sufficient to ensure acceptable reception at the UE. This situation is referred to as a congestion event at the BS. When a congestion event occurs, the BS may have to refrain from processing one or more transport blocks received from the CRNC.
A CRNC should try to avoid the occurrence of congestion events at the BS it controls. Before the CRNC can undertake any congestion-relieving action, however, it has to be aware that congestion events are occurring (or about to occur). Currently, the only mechanism supported by the UMTS standards to make the CRNC aware of the situation at the BS is the reporting of average transmission power from the BS. This mechanism, however, is often insufficient.
As explained previously, the BS may address a congestion event by discarding one or more transport blocks received from the CRNC. If a BS is aggressive in discarding transport blocks, or if the congestion is not caused by a shortage of transmission power, the average transmission power reported by the BS to its CRNC may not increase to a level high enough for the CRNC to trigger a congestion-relieving action. In such situations, the CRNC is not aware that the BS has discarded some of the transport blocks sent to the BS by the CRNC. The CRNC, therefore, may not do anything to address the congestion issue. Furthermore, the BS may continue discarding transport blocks and the effect of this is perceived only at higher layers, such as the radio link control (RLC) sub-layer or higher depending on the RLC mode. This is problematic and, in severe cases, may even cause the RNS to effectively lose control over the quality of service experienced by some users.
A method and system is therefore needed to effectively address congestion by improving the interoperability of an RNC and a BS in cellular systems with downlink congestion control.